WEDNESDAY, July 31 (HealthDay News) -- Millions of Americans head to the tattoo parlor to get "inked" each year, but a new study suggests that getting a tattoo over a mole or birthmark may not be healthy.

That's because having a tattoo over a mole especially can make it difficult to detect the development of skin cancer, the researchers said.

Reporting July 31 in the journal JAMA Dermatology, researchers in Germany pointed to the case of a young man who developed melanoma on a pre-existing colored skin lesion (mole or birthmark) within a tattoo during and between phases of laser tattoo removal.

Sixteen other cases of melanoma developing within tattoos have been reported in English-language journals, the study authors said.

"In general, tattoos should never be placed on pigmented lesions; if they are, the tattoos should never be treated by laser," said the researchers, who were led by Dr. Laura Pohl of Laserklinik Karlsruhe.

Dermatologists in the United States concur that moles should be no-go areas for tattoos.

"Fifty percent of all melanomas develop in pre-existing moles," said Dr. Hooman Khorasani of the Icahn School of Medicine at Mount Sinai, in New York City. "It is harder to do surveillance on moles that are covered by tattoos, as the tattoo ink camouflages the mole and sometimes interferes with some of the tools we use for detection."

Tattoo removal can make mole surveillance more difficult too, he said.

"Once you start the laser removal, the laser can also remove the pigment that the melanoma cells make called melanocytes," Khorasani said. "Therefore, any irregular pigment that one would expect to detect will not be detected as easily. This is the reason that some subtypes of melanoma, called amelanotic melanomas, are more dangerous and aggressive."

"If possible, avoid darker ink tattoos directly over your mole, as they can camouflage your mole and make surveillance more challenging," Khorasani advised patients considering a new tattoo.

If tattoo removal is in order, "always have your moles biopsied before having them treated with any kind of laser," Khorasani said. "If you have many moles under your tattoo, try to see a board-certified dermatologist twice per year instead of once per year," as is recommended for the general population.

The German study authors agreed with Khorasani that regular skin assessments should be conducted while people are undergoing laser tattoo removal. If there are any suspicions about skin cancer, the lesion should be removed before laser tattoo removal begins, they said.

Could tattoos themselves help spur a melanoma? Dr. Doris Day, a dermatologist at Lenox Hill Hospital in New York City, said "it's unlikely that melanoma incidence is higher in tattoos due to the tattoo ink or process."

She agreed, however, with the other experts that "the tattoo may conceal a melanoma that is arising either in previously normal skin or from a mole that is covered by the tattoo ink."